Methods and apparatus for displaying images

ABSTRACT

In one aspect, a method of displaying data is provided. The method comprises obtaining projection data of an object by exposing an object to radiation at a plurality of view angles and detecting at least some of the radiation exiting the object to form the projection data, operating a computer to reconstruct the projection data at a reconstruction resolution to form image data comprising a plurality of voxels representing locations within the object, each of the plurality of voxels being assigned an associated intensity indicative of a density of the subject matter at the respective location, determining a maximum resolution for display, above which variation in intensity between adjacent voxels is not supported by information in the projection data, the maximum resolution being less than the reconstruction resolution, and displaying the image data at or below the maximum resolution.

RELATED APPLICATION

This application is a continuation-in-part (CIP) of U.S. applicationSer. No. 11/595664 (hereinafter the '664 application) entitled “METHODSAND APPARATUS FOR OBTAINING LOW-DOSE IMAGING,” by Stewart, et al., filedNov. 9, 2006, which in turn claims priority to U.S. ProvisionalApplication Ser. No. 60/735140, entitled “PLANAR IMAGING METHODS ANDTECHNIQUES,” filed on Nov. 9, 2005, both applications of which areherein incorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to radiation imaging, and moreparticularly, to displaying image data reconstructed from projectiondata of an object obtained from a plurality of view angles.

BACKGROUND OF INVENTION

Imaging apparatus that utilize relatively high energy radiation such asx-ray and gamma rays are widely used to obtain images of subject mattermore or less opaque to electromagnetic energy in the visual spectrum.For example, x-ray imaging technology has been employed in a wide rangeof applications from medical imaging to detection of unauthorizedobjects or materials in baggage, cargo or other containers. X-rayimaging typically includes passing high energy radiation (i.e., x-rays)through an object to be imaged. X-rays from a source passing through theobject interact with the internal structures of the object and arealtered according to various characteristics of the material (e.g.,transmission, scattering and diffraction characteristics, etc.) whichthe x-rays encounter. By measuring changes in the x-ray radiation (e.g.,attenuation, modifications to the energy spectrum, scatter angle, etc.)that exits the object, information related to characteristics of thematerial, such as the density distribution, may be obtained.

Computer tomography (CT) techniques involve capturing transmitted x-rayinformation from numerous angles about an object being imaged toreconstruct a three-dimensional (3D) volume image of the object. Thedata obtained from each view angle is referred to as projection data orview data and is indicative of the absorption characteristics of theobject in directions related to the respective view angle. CT imagingoften involves obtaining hundreds or thousands of projections to form a3D reconstruction of the projection data, thus requiring the object tobe exposed to relatively large doses of x-ray radiation and/or to (largeor small) doses of radiation over relatively long exposure times. Suchlarge doses and/or imaging times may not be suitable for certain imagingapplications having particular safety and/or time constraints. Forexample, when imaging human tissue, and/or when the imaging procedure isperformed on a routine or frequent basis (such as is often the case inmammography), dose levels and/or exposure times used in conventional CTimaging may exceed that which is more desirable.

To reduce a patient's exposure during breast imaging procedures (e.g.,imaging of the human female breast), conventional mammography is oftenperformed by obtaining only a pair of two-dimensional (2D) radiographicimages of the breast (i.e., each image is reconstructed from a singleprojection of the breast), typically acquired at approximatelycomplementary angles to one another. However, the superposition ofstructure within the breast that occurs when 3D structure is projectedonto two dimensions often obscures the true nature of the structure.This superposition of structure may make it difficult to identify ordetect tissue anomalies. For example, distinct structure in 3D thatoverlaps in 2D may make it difficult to distinguish cancerous subjectmatter from benign subject matter within the breast.

Thus, conventional approaches to providing generally low-dose radiationimaging suffer from images that provide confusing representations ofinternal structures of an object due, at least in part, to theprojection of three-dimensional structure onto one or moretwo-dimensional images. The resulting superposition of distinctstructure located at different levels in 3D makes discerning the actualstructure in a 2D representation difficult, rendering conventionalimaging procedures vulnerable to errors in diagnosis. In mammography,the inability to ascertain the true nature of breast structure mayresult in both significant false negative and false positive rates,leading to potential missed early stage cancers in the case of theformer, or unnecessary trauma to the patient and/or unnecessary hospitalvisits, surgical procedures, etc., in the case of the latter.

To address such problems, it has been proposed to use a selected numberof projections obtained from a plurality of view angles to reconstruct a3D image, while still respecting a relatively low dose budget (e.g.,dose budgets suitable for mammography or other tissue exposures that aregenerally dose limited due to safety concerns). U.S. Pat. No. 6,744,848(hereinafter the '848 patent), entitled “METHOD AND SYSTEM FOR LOW-DOSETHREE-DIMENSIONAL IMAGING OF A SCENE,” describes various methods andapparatus for obtaining 3D images in a relatively low dose environment.In addition, U.S. Pat. No. 5,872,828 (hereinafter the '828 patent),entitled “TOMOSYNTHESIS SYSTEM FOR BREAST IMAGING,” describes variousmethods of reconstructing projection data from a generally limitednumber of view angles to form a 3D image. Both the '848 and '828 patentsare herein incorporated by reference in their entireties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an imaging apparatus suitable for implementingvarious aspects of the present invention;

FIGS. 2A and 2B are diagrammatic illustrations of respective exemplaryview angle configurations, in accordance with embodiments of the presentinvention;

FIG. 3A is a diagram illustrating displaying image data at the sameresolution at which it was reconstructed;

FIG. 3B is a diagram illustrating displaying image data at a lowerresolution than which it was reconstructed, in accordance with someembodiments of the present invention;

FIGS. 5A, 5B and 5C are diagrams illustrating transforming voxelneighborhoods in one, two and three dimensions, respectively;

FIG. 6 is a diagram illustrating transforming a voxel neighborhood usingan averaging transformation;

FIG. 7 is a diagram illustrating transforming a voxel neighborhood usinga maximum intensity value transformation, in accordance with someembodiments of the present invention;

FIG. 8 is a diagram illustrating transforming a voxel neighborhood usinga root mean square transformation, in accordance with some embodimentsof the present invention;

FIG. 9 is a diagram illustrating transforming a voxel neighborhood usinga shifted root mean square transformation, in accordance with someembodiments of the present invention;

FIG. 10 is a diagram illustrating transforming slices of image data fordisplay;

FIG. 11 is a diagram illustrating transforming slices of image data fordisplay, in accordance with some embodiments of the present invention;

FIG. 12 is a diagram illustrating transforming slices of image data fordisplay, in accordance with some embodiments of the present invention;

FIG. 13 is a diagram illustrating transforming slices of image data fordisplay, in accordance with some embodiments of the present invention;

FIG. 14 is a diagram illustrating viewing image data from a plurality ofangles, and preventing the image data from being viewed outside a rangeof angles, in accordance with some embodiments of the present invention;

FIG. 15 is a diagram illustrating obtaining projection data from alimited number of projections across a range to determine a boundaryoutside of which image data should not be viewed, in accordance withsome embodiments of the present invention; and

FIG. 16 is a schematic illustration of a slice of image data.

DETAILED DESCRIPTION

As discussed above, problems of conventional low-dose imaging associatedwith the projection of three-dimensional structure onto one or moretwo-dimensional images, have been addressed by obtaining projection dataat a relatively limited number of view angles to reconstruct a 3D imageof an object exposed to radiation. For example, the '828 and '848patents described various methods of obtaining projection data of anobject in relatively low dose environments, and reconstructing theprojection data to form a 3D image of the object. Because thereconstructed image is in 3D, the structure of the object at differentdepths may not be superimposed on top of one another in a confusingrepresentation (at least not to the same degree as in radiographicimages).

An operator (e.g., a radiologist or other diagnostician) may be able toview the 3D image at different depths to analyze the respectivestructure present there. Such a capability may significantly improve theoperator's ability to distinguish structure, for example, todifferentiate healthy tissue from anomalous tissue such as a tumor. Inparticular, an operator can navigate about the image to examine desiredportions of the object without having structure from other depthsinterfering with the analysis. In this way, 3D images may assist inincreasing the diagnostic quality of the images.

However, in some circumstances, 3D images are displayed in such a way asto be misleading, potentially (and unintentionally) leading tomisdiagnosis. In particular, conventional display methods may provideimage data at a resolution higher than the resolution of the informationavailable in the projection data. For example, certain high frequencyinformation not available in the projection data may be presented in theimage data as an artifact of the reconstruction data. Thus,voxel-to-voxel (or pixel-to-pixel) changes in density values representedat higher resolution than the projection data may therefore beartificial and not attributable to structure of the object.

Radiologists may perceive these changes, and may improperly characterizethe changes as resulting from anomalous subject matter. For example, inbreast imaging, a radiologist may characterize these changes asresulting from micro-calcifications, early stage tumor and/or otheranomalous tissue that may be indicative of cancer. Accordingly,displaying 3D images at resolutions that are higher than supported bythe projection data may actually increase rates of misdiagnosis byallowing the physician to, in a sense, over-interpret the image data.That is, if image data is displayed at too high a resolution, aradiologist may interpret artifacts as variation due to actualstructural features of the object.

Applicant has appreciated that by limiting the display resolution ofimage data, changes in density values not reflective of the projectiondata (i.e., artifacts) may be suppressed (i.e., not displayed) to avoidthe artifacts being interpreted as structure. In some embodiments, imagedata is displayed at a resolution less than the resolution at which itwas reconstructed. For example, the display resolution may be the sameor substantially the same as the actual resolution of the acquiredprojection data. As a result, artifacts in the reconstructed image dataresulting from the artificially high reconstruction resolution may besuppressed to avoid the artifacts from being displayed, and potentiallymis-interpreted.

There are many instances in which it may be desirable to display imagedata at a resolution less than the resolution at which it wasreconstructed. For example, in some circumstances, the monitor, screenor other output device used to display image data may not have thecapability of displaying the image data at full resolution. In addition,for displays with high resolution capabilities, it may be desirable todisplay more than one image simultaneously such that the display canonly accommodate the multiple images at reduced resolutions. Asdiscussed above, Applicant has also appreciated that it may bebeneficial to display image data at lower resolutions even if thedisplay capabilities are sufficient to avoid displaying artifacts of thereconstruction algorithm.

Conventional techniques used in reducing display resolution tend to havedeleterious effects on the image. For example, various averagingtechniques may blur the image, removing high frequency information thatmay be important in the analysis of the image. For example, in medicalimaging, high frequency information is often associated with anomaloustissue that is the subject of the diagnosis. In breast imaging,micro-calcifications, early stage tumors, etc., are often characterizedby relatively high frequency and/or high contrast information.Accordingly, conventional approaches to reducing display resolution mayobscure the very subject matter for which the images are being obtained.

Applicant has identified various techniques for reducing the resolutionof image data for display that may maintain increased image fidelityover conventional techniques. In particular, Applicant has identifiedtechniques for reducing the resolution that obscure less of the salient,high frequency and/or high contrast information. In some embodiments, amaximum intensity value of a neighborhood of pixels is used as therepresentative pixel intensity for the neighborhood. In someembodiments, a root mean square value of a neighborhood of pixels isused as the representative pixel intensity for the neighborhood. In someembodiments, a function is performed on the pixel intensities of aneighborhood, followed by one or more operations to determine a singlepixel intensity for the neighborhood. The inverse of the function maythen be applied to the single pixel intensity to form the representativepixel intensity for the neighborhood.

As discussed above, 3D images may assist in remedying confusingrepresentations that occur when structure at different depths aresuperimposed on one another. To further assist in accurate inspection ofmedical images, Applicant has developed a process, implemented (forexample) in software executing on a computer, that allows an operator tonavigate through a 3D image, for example, by allowing an operator tocontrol the depth at which the image is displayed. In conventionalnavigation controls, structure at different depths may appear anddisappear abruptly as the operator progresses from one depth to another,resulting in a very unintuitive experience for the operator that maylead to incorrect diagnosis. Applicant has developed methods ofdisplaying information such that depth transitions appear more naturaland intuitive.

Following below are more detailed descriptions of various conceptsrelated to, and embodiments of, methods and apparatus according to thepresent invention. It should be appreciated that various aspects of theinvention described herein may be implemented in any of numerous ways.Examples of specific implementations are provided herein forillustrative purposes only. In addition, the various aspects of theinvention described in the embodiments below may be used alone or in anycombination, and are not limited to the combinations explicitlydescribed herein.

As discussed above, conventional CT imaging may be employed to obtain 3Dimages of an object. However, full CT imaging requires subjecting anobject to hundreds or even thousands of exposures. Accordingly, CTimaging may be unsuitable for imaging human tissue, and/or performingregular or frequent imaging procedures on human subjects (e.g., breastimaging). The '848 patent describes various methods of obtaining 3Dimages, while limiting the exposure to radiation dose levels suitablefor imaging human tissue.

FIG. 1 illustrates one embodiment of an imaging system 100 suitable forobtaining projection data suitable for forming 3D images in a relativelylow-dose environment, in accordance with various aspects of the presentinvention. Imaging system 100 may be suitable for obtaining projectiondata and reconstructing images according to the various methodsdescribed in the '848 patent and/or the '664 application.

Imaging system 100 includes a radiation source 120, a detector 130, amotion controller 140, an image processor 160 and a display 190. Theimaging system 100 can be used to image a single object 110 or aplurality of objects located within an exposure area 114. The exposurearea 114 defines generally the region of space between the radiationsource 120 and the detector 130, and is located in the path of theradiation provided by radiation source 120 in the direction of detector130. The exposure area 114 may be the entire region of space located inthe path of the radiation passing from the radiation source 120 to thedetector 130, or only a predetermined portion of the space.

Radiation source 120 may be any component or combination of componentscapable of emitting radiation such as x-ray or gamma radiation. Inimaging system 100, radiation source 120 is positioned to emit radiationtoward exposure area 114 such that, when object 110 is present inexposure area 114, at least some of the radiation impinges on object110. In particular, the radiation source 120 is adapted to emitradiation to form a radiation field 116, which may be of any shape orsize. In a preferred embodiment, radiation field 116 is a beam thatradiates outward from a focal point of radiation source 120substantially in the shape of a cone, that substantially encloses object110 within a cone of x-rays during exposures. However, radiation field116 may form other shapes such as a fan beam, pencil beam, etc., and maybe arranged to expose any portion of object 110, as the aspects of theinvention are not limited in this respect.

Radiation source 120 may be capable of being moved about object 110 suchthat radiation may be directed at object 110 from a plurality of angularpositions, i.e., a plurality of view angles with respect to object 110(e.g., as described in further detail below). Detector 130 is positionedto receive at least some of the radiation that passes through theexposure area 114, and in particular, radiation that has penetrated andexited object 110. Detector 130 may be a single detector, or a detectorarray disposed continuously or at a plurality of discrete locations.Detector 130 may be formed from any type of material responsive toradiation generated by radiation source 120. In response to impingingradiation, detector 130 produces signals indicative of the intensity ofradiation impinging on the detector surface. Accordingly, recordedintensities of radiation passing through the object as represented bythe detector signals carry information about the absorptioncharacteristics of object 110, and form, at least in part, projectiondata of object 110.

Detector 130 may be configured to be moved in correspondence with theradiation source 120 to detect radiation exiting object 110 from theplurality of view angles. Motion controller 140 may be coupled toradiation source 120 and detector 130 to cause the rotational movementof the radiation source/detector apparatus such that, as the apparatusrotates about the object, the object remains positioned within theexposure area between the source and detector. Motion controller 140 maybe capable of being programmed to move the radiation source and detectorto any desired view angle with respect to object 110. Together, theradiation source 120, detector 130 and motion controller 140 permitprojection data of object 110 to be obtained from any set of viewangles. In some embodiments, motion controller 140 may be programmed tocontrol the position of the radiation source and detector independently.For example, the motion controller may move the radiation source anddetector along different paths as projection data is obtained from thedifferent view angles, as the aspects of the invention are not limitedin this respect.

In another embodiment, the detector 130 remains stationary as theradiation source is moved about the object. For example, if the detector130 is sufficiently large (e.g., a flat panel two-dimensional detectorarray) and/or if the angular range over which projection data isobtained is sufficiently small (e.g., the angular range is limited to arange between 5° and 45° both clockwise and counterclockwise from areference view angle), a single position for the detector 130 may besufficient to capture projection data from each of the desired viewangles. In addition, in embodiments where detector 130 remainsstationary, the object may be positioned in direct contact with thedetector.

At each orientation, referred to as a view angle, the detector signalgenerated by each detector in the array indicates the net totalabsorption (i.e., attenuation) incurred by material substantially in aline between the radiation source and the detector. Therefore, the arrayof detection signals at each view angle records the projection of theobject onto the detector array at the associated view angle. Forexample, using a 2D detector array, the resulting detector signalsrepresent the 2D density projection of the object on the detector arrayat the corresponding view angle. The signals generated by the detectorsform, at least in part, projection data (or view data) of the object.

Projection data obtained from multiple view angles about the object maybe used to compute a density distribution of the object (i.e., todetermine density values for locations within the object). The processof converting projection data (i.e., attenuation or transmission as afunction of view angle) to density data (i.e., density as a function oflocation within the object) is referred to as reconstruction. That is,density values are reconstructed from information contained in theprojection data. Typically, density values are expressed as image data,i.e., pixel or voxel intensities in two-dimensional (2D) andthree-dimensional (3D) images, respectively.

Image processor 160 may be configured to reconstruct the projection datato form images of the object (e.g., 2D or 3D images of the object).Image processor 160 may be configured to implement any desiredreconstruction algorithm capable of mapping recorded radiation intensityvalues (e.g.., detector signals from detector 130) to correspondingdensity values at a desired resolution. Image processor 160 may also beconfigured to automatically process reconstructed images to, forexample, reduce the resolution of the image data for display, transformreconstructed image data to display image data to facilitate imagenavigation, etc., as described in further detail below. It should beappreciated that image processor 160 may be configured to carry out anycomputation and/or combination of computations described herein, as theaspects of the invention are not limited in this respect.

Image processor may be one or more processors located proximate orremote from the radiation source and detector. The image processor maybe configured to execute programs stored on a computer readable mediumsuch as a memory, accessible by the image processor. Image processor maybe part of a computer or computer system capable of receiving projectiondata. Imaging system 100 may also include a display 190, such as amonitor, screen and/or other display device capable of presenting apixel representation of reconstructed image data (e.g., display imagedata). It should be appreciated that the above described components aremerely exemplary, and any suitable imaging apparatus of anyconfiguration and/or combination of components may be used to implementany one or combination of the methods described above, as the aspects ofthe invention are not limited in this respect.

Projection data may be obtained in many different ways. As discussed inthe '848 patent, radiation exposures may be performed at a number ofnon-uniformly distributed view angles. For example, the change in anglefrom one view angle to another may increase as the angle from areference view angle (e.g., position A in FIG. 1) increases. That is, asa radiation source is rotated about an object from a reference position,the angle between successive exposures may be increased. However, thevarious view angles selected also may be uniformly distributed, as theaspects of the invention are not limited in this respect. FIGS. 2A and2B illustrate exemplary methods of obtaining projection data of anobject from a plurality of view angles.

In FIG. 2A, the plurality of view angles used to obtain projection dataof object 210 are distributed with non-uniform angular offsets withrespect to one another. For example, as the view angles are rotated awayfrom a reference view angle at θ₀=0° in both the clockwise andcounterclockwise directions, the angle between each successive viewangle increases. In particular, in the clockwise direction(θ₁-θ₀)<(θ₂-θ₁)<(θ₃-θ₂), etc. Similarly, in the counterclockwisedirection, (θ_(1′)-θ₀)<(θ_(2′)-θ_(1′))<(θ_(3′)-θ_(2′)). As discussed inthe '848 patent, performing exposures at non-uniform angles mayfacilitate obtaining optimal projection data for a given dose budget. Itshould be appreciated that the number and distribution illustrated inFIG. 2A are merely exemplary.

Any number of view angles may be used at any desired distribution, asthe aspects of the invention are not limited in this respect. Moreover,the view angles need not be distributed symmetrically with respect tothe reference view angle, as any desired distribution may be used withthe various aspects of the invention. Furthermore, the total angularrange of the view angles at which projection data is obtained need notbe equal to 180°, but may cover any desired range. For example, theangular range could cover as little 5° or less or could be any range upto and including 360°.

In FIG. 2B, the angular offsets are distributed essentially uniformlyabout object 210. For example, as the view angles are rotated away fromthe reference view angle at θ₀=0° in both the clockwise andcounterclockwise directions, the angle between each successive viewangle remains essentially the same. In particular, in the clockwisedirection, (θ₁-θ₀)=(θ₂-θ₁)=(θ₃-θ₂), etc. Similarly, in thecounterclockwise direction, (θ_(1′)-θ₀)=(θ_(2′)-θ_(1′))=(θ_(3′)-θ_(2′)).Accordingly, any number of view angles may be distributed in anyfashion; uniformly or non-uniformly, symmetric or asymmetric, etc., asthe aspects of the invention are not limited in this respect. Asdiscussed above, the angular range over which projection data isobtained need not be 180° as illustrated in FIGS. 3A and 3B, but maycover a range greater than or less than 180°, as discussed in furtherdetail below.

Projection data obtained according to the methods described above mayhave different resolutions along the different axes (i.e., asymmetricresolution). In particular, because projection data is obtained at fewerview angles than in full CT (e.g., between 15-50 view angles versushundreds or even thousands of view angles), the resolution in thez-direction (see e.g., the coordinate frame in FIG. 1) may besubstantially less than in the x-direction and y-direction. That is,because less information is available along the z-axis, reconstructionmay be unable to accurately assign density values at the same resolutionachievable in the XY plane (also referred to as the in-plane).

The resolution in the x-direction and y-direction may be largely afunction of the resolution of the detector array and the operatingparameters of the radiation source. For example, each detector ordetector location capable of being sampled for a detection signal maycorrespond to a pixel in the resulting image (e.g., a pixel in a sliceof a 3D image through the XY plane). In addition, the resolution in theXY plane may also be a function of the radiation intensity, radiationenergy and/or radiation field density of the radiation emitted from theradiation source. The resolution in the XY plane is substantiallyindependent of the number of view angles from which projection data isobtained.

However, each pixel in the z-direction is determined fromtomosynthetically computing information from projection data obtainedfrom multiple view angles. As a result, the resolution in thez-direction (also referred to as the out-of-plane direction) may beincreased by obtaining projection data from an increasing number of viewangles. That is, increasing the angular range and decreasing the spacingbetween successive view angles at which projection data is obtained eachcontribute to the resolution in the z-direction. However, as discussedabove, the greater number of view angles at which projection data isobtained, the greater the exposure of the object to radiation and thelonger the acquisition time. Some imaging applications may be limited toa particular radiation dose-budget guided by safety and/or timeconstraints, thereby limiting the number of view angles at whichprojection data should be obtained. As a result, in many applications,the in-plane resolution will be greater than the out-of-planeresolution.

As discussed above, reconstruction involves transforming projection data(e.g., attenuation information as a function of view angle) into imagedata (e.g., density values as a function of location). While there aremany different methods of performing reconstruction, the methods performthe same fundamental operation of mapping intensity values recorded atthe detectors to density values at discrete locations in space (e.g.,mapping detector signals to values in an image that represent 2D or 3Dspace). The reconstruction algorithm may be configured to map values toa space partitioned into volumes of any size, which may be the same ordifferent than the actual resolution of the projection data from whichthe image data is determined.

The actual resolution relates to the amount of information in theprojection data. The reconstruction resolution relates to how finelyspace is partitioned for reconstruction (i.e., how small are the logicalvolumes representing discretized space, each of which are assigned adensity value). The display resolution refers to the resolution at whichimage data is displayed on, for example, a monitor, screen or otherdisplay device. Projection data is often reconstructed at a resolutionhigher than the information available in the projection data (i.e., thereconstruction resolution is greater than the actual resolution).

In some instances, reconstructing at higher resolutions than the actualresolution may be necessary to account appropriately for the geometry ofthe acquisition process. As a result, at least some of the informationin the image data is artificial (e.g., it has no physical basis in theprojection data and is therefore an artifact of the reconstructionprocess). Conventionally, image data is displayed at the reconstructionresolution, thus the reconstruction artifacts are similarly displayed.However, Applicant has appreciated that changes in density values (e.g.,intensity variation) at artificially high resolutions, when displayed,may be perceived by radiologists and may be interpreted as, for example,tissue anomalies that may lead to misdiagnosis. By limiting the displayresolution, density variation at resolutions higher than the actualresolution may be suppressed, preventing corresponding artifacts fromthe reconstruction process from being displayed to a viewer.

FIG. 3A illustrates a conventional technique for viewing display data.For example, projection data 305 may have been obtained by exposing anobject to radiation from a plurality of view angles. The projection data305 has an actual resolution related in part to the geometry of thedetector array and radiation emission parameters (e.g., the in-planeresolution) and the number and position of view angles from which theobject was obtained (e.g., the out-of-plane resolution). The projectiondata may have been obtained by performing exposures at a relativelysmall number of view angles (e.g., substantially fewer view angles thanneeded for full CT) to satisfy desired dose and/or time constraints.Accordingly, projection data 305 may have an asymmetric resolution(e.g., the in-plane and out-of-plane resolutions may be different).

A reconstruction 310 may be performed on projection data 305 to formreconstructed image data 315. The reconstruction may be performedaccording to a particular reconstruction resolution, illustratedschematically by the size of the volume elements (voxels) by which imagedata 315 is partitioned. The reconstruction algorithm may be configuredto assign each voxel a density value (referred to as the intensity ofthe voxel), based on the information in the projection data. As shown,the reconstruction resolution in the XY plane is greater than theresolution in the z-direction (i.e., the partitioning in the XY plane issmaller than the partitioning in the z-direction). This may be partiallydue to the fact that projection data was obtained from a relativelysmall number of view angles (e.g., from between 1-30 view angles).

In FIG. 3A, reconstruction 310 may be configured to reconstruct imagedata at a resolution higher than the actual resolution of the projectiondata. For example, reconstruction at a higher resolution may benecessary to appropriately reconstruct the projection data at theresolution and geometry at which it was obtained. As a result,reconstruction 310 may assign different density values to adjacentvoxels even though information about density changes at that resolutionis not available in the projection data. That is, the projection datamay not contain enough information to distinguish density at theresolution of the reconstruction. Accordingly, some variation in densityin the reconstructed image data may be artifacts of reconstruction,rather than an accurate rendering of the imaged object.

Conventionally, image data is displayed at the same resolution as it wasreconstructed. For example, display procedure 320 may displayreconstructed image data 315 at the same resolution, as shownschematically by display image data 325. Conventional understanding isthat image data should be displayed at the highest resolution possibleto display the maximum amount of information. For example, theconventional belief is that the higher resolution display data providesricher information on which a radiologist can perform a diagnosis.However, it may be advantageous to display image data at a resolutioncommensurate with eliminating at least some reconstruction artifacts,typically less than the maximum available resolution in thereconstructed image data.

As illustrated, a three-by-three voxel neighborhood 317 is shown withexemplary density values shown as greyscale intensities (i.e., voxelintensities). However, the projection data may not have the resolutionto distinguish different density values at this high a resolution, andat least some of the variation in density values shown is an artifact ofthe reconstruction process. When the image data is displayed at the sameresolution, the variation in density values is perceptible (seeneighborhood 317′), even though the variation is not physicallysupported in the projection data. A radiologist may view this variationand interpret the variation as some sort of structure or feature in theimage (e.g., as a tissue anomaly). In a breast imaging procedure, forexample, the lighter intensity at the center of neighborhood 317′ may beinterpreted as a micro-calcification, early stage tumor, etc., eventhough the variation that gave rise to the intensity may be artificial.

By limiting the display resolution, density variation at resolutionshigher than the actual resolution may be suppressed, preventingcorresponding artifacts from the reconstruction process from beingdisplay to a viewer. Limiting the displayed resolution to substantiallythe actual resolution may reduce the opportunity for a radiologist tomisinterpret reconstruction artifacts as salient structure in the image.In some embodiments, the display resolution is limited so that variationis not displayed at resolutions higher than are supported by theprojection data.

FIG. 3B illustrates concepts related to limiting the display resolution,in accordance with some embodiments of the present invention. In FIG.3B, the projection data 305, and reconstruction image data 315 may besimilar to that shown in FIG. 3A. Accordingly, the projection data maybe reconstructed at a resolution higher than the actual resolution.However, rather than displaying the image data at the reconstructionresolution, the display procedure 320′ displays the image data 325′ at areduced display resolution, at least with respect to the z-axis wherethe actual resolution is particularly limited and therefore more likelyto result in image data artifacts after reconstruction. In someembodiments, the resolution reduction is performed so that the displayimage has no variation not accounted for in the projection data. As aresult, any variation in the display image data will be a result of andsupported by information in the projection data.

The display resolution may be reduced by, for example, consideringdensity values in a neighborhood of voxels and computing a singledensity value from the neighborhood. The size of the neighborhood may beselected in view of the amount of resolution reduction required. Inaddition, the neighborhood selected to transform into a single voxeldensity value may be chosen in any direction, even if a reduction inresolution is not desired along the corresponding axis. As shown by thevoxel sizes in FIG. 3B, the resolution in the z-direction is less forthe display image data 315 than for the reconstruction image data 325.The resolution reduction in the z-direction may be achieved, forexample, by averaging three adjacent voxel density values in thez-direction to produce a single voxel having the computed average as itsdensity value to reduce the resolution in the z-direction by a factor ofthree.

Resolution reduction may be achieved by selecting any size and/ordimensioned neighborhood, and performing any type of computation on theneighborhood, as the aspects of the invention are not limited in thisrespect. As illustrated, the density variation in the z-direction in thereconstructed image data (e.g., artifacts from the reconstruction) aresuppressed in the display image data, thus preventing the artificialvariation from being misinterpreted by a viewer of the image data (e.g.,a radiologist analyzing and/or otherwise diagnosing the image data).

FIG. 4 illustrates a method of reducing the display resolution, inaccordance with some embodiments of the present invention. In act 410,projection data is obtained by exposing an object to radiation at aplurality of view angles. In some embodiments, the projection data isobtained from a relatively limited number of view angles (e.g., between1-30 view angles distributed uniformly or non-uniformly about theobject) to satisfy desired safety and/or time constraints. Accordingly,in some embodiments, the projection data may have an asymmetric actualresolution (e.g., the in-plane resolution may be higher than theout-of-plane resolution). However, the projection data may be obtainedin any manner, as the aspects of the invention are not limited in thisrespect.

In act 420, the projection data is reconstructed at a resolution higherthan the actual resolution along at least one axis. For example, theprojection data may be reconstructed at a resolution appropriate forreconstructing asymmetric resolution projection data obtained at aparticular geometry. The reconstruction may be performed according toany desired reconstruction algorithm capable of assigning density valuesto voxels at the reconstruction resolution. In some embodiments, thereconstruction resolution is asymmetric due to, for example, acquiringthe projection data at a relatively small number and range of viewangles and the reconstruction resolution is higher than the actualresolution along the asymmetric axis only (e.g., the out-of-planeresolution).

In act 430, a maximum resolution is determined, along at least one axis,for the display resolution such that artificial variation in the densityvalues is substantially suppressed. For example, the maximum resolutionmay correspond to the maximum resolution supported by the projectiondata. This maximum resolution may be determined by considering thegeometry of the imaging equipment, the number and distribution of viewangles from which the projection data was obtained, the parameters ofthe emitted radiation, etc. For image data having asymmetric resolution,the maximum resolution may be different along each axis of thereconstructed image data. There may be one or more axes over which themaximum resolution is the same or substantially the same asreconstruction resolution. For example, the maximum in-plane resolutionmay be the same as the in-plane reconstruction resolution, while themaximum out-of-plane resolution may be less than the out-of-planereconstruction resolution.

In act 440, the image data is displayed at or below the maximumresolution along each axis of the image data. The resolution reductionmay be achieved by any method, some exemplary methods of which aredescribed in further detail below, without limitation. By displaying theimage data at or below the determined maximum resolution, some of thedensity variation resulting from high resolution reconstruction may besuppressed, preventing that particular variation from being displayedand potentially misinterpreted by a viewer of the image.

As discussed above, there may be a variety of reasons to reduce thedisplay resolution of image data. For example, the display resolutionmay be reduced to facilitate the prevention of false high resolutionvariations from being displayed to a viewer, as discussed in theforegoing. In addition, it may be desirable to reduce the displayresolution to display the image data on lower resolution screens,monitors or other displays. Moreover, it may be desirable to reduce thedisplay resolution so that multiple images may be simultaneouslydisplayed. However, conventional methods of reducing the displayresolution may obscure salient or otherwise important information. Forexample, conventional averaging techniques (such as the above-mentionedvoxel averaging technique) tend to remove high frequency informationthat may be important to accurate medical diagnosis, or other imageanalysis.

FIG. 5A illustrates a method of reducing the display resolution of imagedata in one dimension. Image data 515 represents a portion of an imageof an object obtained from exposing the object to radiation from aplurality of view angles. The image data comprises a plurality of pixels(generally in 2D) or voxels (generally in 3D) represented as cells in anarray, each having a value (referred to as intensity) indicative of thedensity of the object at a location associated with the respectivevoxel. The term “intensity” refers herein to any vector or scalar valuethat indicates relative degree. To reduce the resolution, a collectionof (usually contingious) voxels, referred to as a neighborhood may betransformed into a single voxel having an intensity representative ofthe neighborhood. Thus, each neighborhood of voxels may be reduced to asingle voxel, thus reducing the resolution.

In FIG. 5A, the display resolution is reduced by a factor of five in thez-direction, from image data 515 to reduced resolution display imagedata 525. Accordingly, the image data may be grouped into neighborhoodsof contiguous voxels in the z-direction. Twenty-five such neighborhoodsare shown in FIG. 5A, each voxel being labeled with the number of theneighborhood with which it is associated. As discussed above, resolutionreduction may be achieved by transforming a neighborhood of voxels to asingle voxel, and more particularly, transforming the density values ofa neighborhood to a single representative density value, illustrated bytransformation 520 in FIG. 5. Thus in the reduced resolution image data525, each neighborhood 1-25 is represented by a single voxel labeledwith the respective neighborhood, and having an intensity representativeof the neighborhood, as discussed in further detail below.

It should be appreciated that resolution reduction may be performed inany number of dimensions. For example, FIG. 5B illustrates a resolutionreduction by a factor of three in two dimensions. In particular, imagedata 515′ is divided into a plurality of two-dimensional neighborhoodslabeled 1-16. Image data 515′ is transformed into image data 525′ at thereduced display resolution by transforming the neighborhood densityvalues to a respective representative density value according totransformation 520′. FIG. 5C illustrates a resolution reduction by afactor of two in three dimensions. In particular, image data 515″ isdivided into a plurality of three-dimensional neighborhoods. Image data515″ is transformed into image data 525″ at the reduced displayresolution by transforming the neighborhood density values to arespective representative density value according to transformation520′.

It should be appreciated that resolution reduction need not be the samein every direction when performed in multiple dimensions, as the aspectsof the invention are not limited in this respect. For example, withimage data having an asymmetric resolution, the resolution reduction ineach direction may be different (e.g., the in-plane resolution may bereduced by a smaller factor than the out-of-plane resolution withrespect to the reconstruction resolution). In addition, neighborhoodsmay be chosen to be of any size or shape and are not limited toincluding contiguous voxels in the one or more directions in which theresolution is to be reduced.

In conventional resolution reduction, the transformation fromreconstruction image data to display image data is often an averagingoperation on the neighborhood intensities, as shown by the exemplarytransformation 620 illustrated in FIG. 6. In particular, FIG. 6illustrates a neighborhood 615 which may be a portion of reconstructionimage data. Each voxel is labeled with its associated density value.Transformation 620 takes the average of the neighborhood intensities toform a representative voxel 625 having the average as its density value(i.e., the representative intensity for the neighborhood is the averageof the neighborhood intensities).

However, simple averaging may obscure salient information or features inthe object (e.g., has the effect of applying a low-pass filter to theneighborhood). For example, in the neighborhood 615, there is a clusterof relatively high density material in the bottom left hand corner thatmay be related to important information. The averaging, however,suppresses significant information regarding this high density clusterby considering uniformly the contributions from each voxel in theneighborhood, removing some of the high frequency information during thedisplay procedure.

Numerous transformations that may be more suitable for resolutionreduction. In some embodiments, for example, the maximum intensity value(MIV) in a neighborhood may be selected as the representative intensity,as shown by the exemplary transformation illustrated in FIG. 7. Inparticular, neighborhood 715 may be image data from a portion of animage to be displayed. Transformation 725 takes the maximum intensityvalue of the neighborhood (202) and assigns the MIV as therepresentative intensity to voxel 725. In some embodiments, a functionis applied to the neighborhood to transform the intensities, followed byone or more operations to convert the neighborhood intensity values to asingle intensity value. The inverse of the function applied to transformthe neighborhood may then be performed on the single value to transformthe single value into the representative intensity assigned to thesingle voxel representing the neighborhood.

FIG. 8 illustrates a using a power function, an average operation and aninverse power function (root function) to determine a representativeintensity value for a neighborhood (e.g., to perform a root mean squaretransformation). In particular, function 822 transforms neighborhood 815to neighborhood 817 by taking the square of the intensity values in theneighborhood. Operation 823 transforms neighborhood 817 into a singleintensity value 819 by taking the average of the squared intensities,and inverse function 824 transforms the single intensity value 819 intothe representative intensity value 825 by taking the square root of theaverage. Accordingly, the intensity value 157 is root mean square of theneighborhood of intensities (rounded to the nearest integer). The rootmean square weights higher density values with more significance (i.e.,via the power function), which may be associated with subject matter ofinterest in an image. In some instances, the root mean squaretransformation avoids the blurring effect of pure averaging byemphasizing the contribution of higher intensity values.

FIG. 9 illustrates another method of transforming a neighborhood into asingle intensity value to reduce the resolution, in accordance with someembodiments of the present invention. The method shown in FIG. 9 issimilar to the method illustrated in FIG. 8 in that a root mean squareoperation is performed. However, prior to squaring the intensity values,function 920 shifts the intensity values by subtracting an offset fromthe neighborhood. After the offset has been subtracted from eachintensity value in the neighborhood, the intensity values may be squared(transformation 921), and the average taken of the shifted and squaredintensity values (transformation 922). The square root of the averagemay be performed (transformation 923), and the offset added back to theintensity value to form the representative intensity value for theneighborhood (transformation 924).

In some embodiments, the offset is related to a characteristic densityof the object being imaged. By subtracting the offset from theneighborhood intensities before squaring, differences from thecharacteristic density are accorded even further significance in thetransformation. In breast imaging, the density of the fatty tissue thatcomprises most of the breast material may be subtracted off from theneighborhood before performing the root mean square. In breast imaging,for example, it may be important to determine density anomalies withreference to the predominant surrounding tissue. By subtracting offdensity values characteristic of healthy breast tissue, the remainingvalues relative to the fatty tissue may be further emphasized. As aresult, the removal of important information that often results frompure averaging may be mitigated in this respect. Other functions may beperformed on the neighborhood before averaging to facilitate selecting arepresentative intensity value without obscuring important informationin the image, as the aspects of the invention are not limited in thisrespect.

As discussed above, confusion resulting from the superposition ofstructure at different depths may be reduced by providing 3D images. Inparticular, the various methods described in the '828 and '848 patentsmay be used to provide 3D image data that can be viewed at differentdepths without structure from other depths obscuring the display. Anoperator may navigate through a 3D image, for example, by allowing anoperator to control the depth at which the image is displayed. Inconventional navigation controls, structure at different depths mayappear and disappear abruptly as the operator progresses from one depthto another, resulting in a very unintuitive experience for the operatorthat may lead to incorrect diagnosis. Methods of displaying informationsuch that depth transitions may appear more natural and intuitive may bebeneficial.

FIG. 10 illustrates one method of transforming reconstruction data to bedisplayed as an operator navigates through the image data. As discussedabove, it may be beneficial to display data at a lower resolution thanthe resolution at which it was reconstructed. For example,reconstruction image data 1015 may be reconstructed at a reconstructionresolution having a single unit for each pixel in the z-direction (e.g.,1 mm slices). Each rectangle in the reconstruction data represents aslice of image data in the XY plane. The resolution in the XY plane isnot illustrated.

The term “slice” refers to a planar section of image data havingdimensions in voxel units, with a single voxel in one of the dimensions(e.g., a slice may have dimensions N×M×1 voxels for a 3D slice). Forexample, FIG. 16 illustrates an exemplary slice 1615 comprising aplurality of voxels 1615 a, the slice having dimensions 20×15×1 voxelsin the x-direction, the y-direction and the z-direction, respectively.It should be appreciated that N and M may be any number and may be ofthe same or different value. In general, each voxel in a slice has anassociated intensity, for example, indicative of a density value for alocation in space represented by the corresponding voxel.

Display image data 1025 is being displayed at a display resolution thathas been reduced by a factor of five. As with the reconstruction imagedata, each rectangle in the schematic representation of display imagedata 1025 represents a slice of display image data in the XY plane. Eachslice of display data may be generated by transforming (e.g.,transformations 1020 a-920 e) intensity values in the corresponding fiveslices (e.g., a five slice neighborhood) to obtain representative pixelvalues. For example, to display image data in the first slice of displayimage data 1025, the intensities values in slices 1-5 of reconstructionimage data 1015 may be transformed by transformation 1020 a to arrive atpixel intensities representative of pixel intensities in a five-sliceneighborhood. Similarly, the intensities in the second slice of displayimage data 1025 may be transformed from slices 6-10 of thereconstruction image data, and so on down through the image. Thetransformation may be an average, a maximum intensity value, a root meansquare, or any other transformation (e.g., any one or more combinationof transformations described above) that transforms neighborhood densityvalues to representative density values.

When an operator navigating through the image data navigates, forexample, from slice 1 to slice 2 of display image data 1025, the pixelintensities change because pixel intensities in slice 1 of display imagedata 1025 are computed using reconstruction slices 1-5 of reconstructionimage data 1015, and the pixel intensities of slice 2 of display imagedata 1025 are computed using reconstruction slices 6-10 ofreconstruction image data 1015. As a result, as the intensities change,structure may appear and disappear abruptly as the operator navigates upand down through the display image data, providing an unnatural andunintuitive display of the image data. For example, structure justbeyond the current viewing depth may be invisible until abruptlyappearing as the operator scrolls to the next discrete slice of thedisplay data. The abrupt changes may make it difficult to synthesize howthe structure at the different depths are associated and/or may makediagnostic analysis of the images confusing.

Applicant has appreciated that by blending in pixel intensity fromreconstruction slices as an operator navigates through an image, thedisplay viewed by the operator may be more intuitive and facilitateeasier and more accurate diagnosis. FIG. 11 illustrates a method ofdisplaying image data as an operator navigates around the image, inaccordance with some embodiments of the present invention. As in FIG.10, reconstruction image data may be reconstructed at a reconstructionresolution having 1 unit slices in the z-direction, and displayed at adisplay resolution reduced by a factor of five (e.g., 5 unitresolution). However, as an operator navigates in the z-direction, pixelintensities get transformed according to a sliding window that blends inslices at the reconstruction resolution, rather than at the displayresolution.

For example, the pixel intensities of slice 1 of display image data 1125may be computed from slices 1-5 of the reconstruction image data (i.e.,transformed from pixel intensities in slices within window 1130 a).However, as the image data is viewed at increasing depths, the pixelintensities of the reconstruction data get rolled in as the data isdisplayed at increasing depths. Thus, as an operator navigates deeper byone unit of reconstruction resolution, pixel intensities in sliceswithin window 1130 b (which slides in conjunction with the operator'snavigation depth) are transformed to generate representative pixels inthe corresponding slice of the display image data (e.g., the pixelintensities are transformed from slices 2-6 of the reconstruction imagedata). Accordingly, the window from which pixel intensities aretransformed follows the navigation up and down through the data at thereconstruction resolution, to effect more gradual changes in intensity.The smoother transition may assist in better synthesizing informationfrom different slices, and may facilitate a more intuitive viewingexperience that aids in more accurate diagnosis. It should beappreciated that pixel intensities from slices within the window may betransformed according to any transformation, as the aspects of theinvention are not limited in this respect.

In FIG. 12, illustrates another method of transforming reconstructiondata to display data during image navigation, in accordance with someembodiments of the present invention. The method in FIG. 12 may besimilar with respect to FIG. 11 in that a sliding window is used totransform reconstruction image data to display image data at a reducedresolution. In particular, as an operator navigates to a new depth at ascale of the reconstruction resolution, pixel intensities from sliceswithin the sliding window are transformed to representative pixelintensities at the reduced resolution.

However, window 1230 in FIG. 12 may consider a larger number of slices.In particular, in FIG. 11, window 1130 transformed pixel intensitiesfrom a number of slices equal to the resolution reduction factor (e.g.,window 1130 transformed pixel intensities from 5 single unit slices toachieve a resolution reduction by a factor of five). In FIG. 12, aresolution reduction by a factor of five may still be achieved, however,window 1230 considers pixel intensities from more than the correspondingfive slices (i.e., from more than the corresponding neighborhood). Inparticular, window 1230 considers an additional slice on both sides ofthe corresponding neighborhood. However, to avoid having the pixelintensities outside of the neighborhood contribute too significantly,the pixel intensities from slices inside and outside of the neighborhoodmay be weighted differently.

In FIG. 12, pixel intensities from slices inside the neighborhood fullycontribute (i.e., have a weighting of 1), while pixel intensities fromslices outside the neighborhood are weighted by 0.5. Applicant hasappreciated that by considering pixel intensities outside of theneighborhood at a reduced weighting, structure just outside theneighborhood may be partially visible, enhancing the smoothness ofstructure transitioning in and out of view as an operator navigatesthrough the image data. For example, an operator positioned at theillustrated depths in FIG. 12 will perceive (though perhaps faintly)intensity resulting from structure outside the five slice neighborhood(e.g., from slice 9 and 15). As the operator continues to navigatedownward, the intensity contribution from slice 9 disappears, and theintensities from slice 15 transitions into the neighborhood and is fullyweighted. In addition, pixel intensities from slice 10 and 16 are now onthe periphery and contribute at half-weight. As a result, structure atdifferent depths in the object may transition into and out of view moresmoothly, making navigation of the image data more natural andintuitive.

It should be appreciated that the weighting scheme used in FIG. 12 ismerely exemplary, and any weighting scheme may be used (e.g., pixelintensities outside the neighborhood may be weighted by any desiredamount). In addition, more than one peripheral slice on each side of theneighborhood may be considered, as the aspects of the invention are notlimited in this respect. In particular, the window may be of any shapeand size to incorporate a desired number of slices and any desiredweights. For example, FIG. 13 illustrates a window 1330 that includestwo peripheral slices on both sides of the neighborhood, treating eachperipheral slice with a decreased weighting. In some embodiments, pixelintensities within the neighborhood are also weighted. For example, awindow may be shaped like a triangle window or a Hanning window thatweights pixel intensities of slices towards the center of the windowwith more significance than pixel intensities of slices more towards theperiphery.

Conventional software that allows a user to navigate through 3D imagedata typically allows motion in a single direction. For example,conventional software may limit an operator to viewing image data in theXY plane at successive slices. However, conventional software may notallow an operator to view the 3D data at different angles. FIG. 14illustrates schematically 3D image data 1415. Conventional displaysoftware may allow an operator to view the image data in the XY planefrom a direction substantially perpendicular to the plane. For example,an operator may view the XY plane from direction 1440 a, and bepermitted to view slices at successive depths in this direction only.However, some software may allow the user to view the image data frommultiple views (e.g., to view the data from directions 1440 b, 1440 b′,1440 c, 1440 c′, etc.), and permit navigation through the image dataalong those views.

However, in some instances, an operator should be prevented from viewingdata at angles in which projection data was not obtained. For example,FIG. 15 illustrates an exemplary set of view angles from whichprojection data was obtained. Because projection data was not obtainedat angles beyond angles θ₃ and θ_(3′), and operator should not beallowed to view the data at angles beyond the boundaries at which theprojection data was obtained to avoid displaying data that has nosupport in the projection data (e.g., to avoid displaying data that isartificial). Referring back to FIG. 14, in some embodiments, displaysoftware programmed to allow an operator to navigate through image datain multiple directions is configured to prevent an operator from viewingthe image data outside angles from which projection data was obtained.For example, the display software may prevent an operator from viewingdata from angles outside boundaries 1450 a and 1450 a′, to preventdisplaying significant amounts of data that is artificial andpotentially misleading to the operator, for example, a radiologistperforming a diagnosis on the image.

The above-described embodiments of the present invention can beimplemented in any of numerous ways. For example, the embodiments may beimplemented using hardware, software or a combination thereof. Whenimplemented in software, the software code can be executed on anysuitable processor or collection of processors, whether provided in asingle computer or distributed among multiple computers. It should beappreciated that any component or collection of components that performthe functions described above can be generically considered as one ormore controllers that control the above-discussed function. The one ormore controller can be implemented in numerous ways, such as withdedicated hardware, or with general purpose hardware (e.g., one or moreprocessor) that is programmed using microcode or software to perform thefunctions recited above.

It should be appreciated that the various methods outlined herein may becoded as software that is executable on one or more processors thatemploy any one of a variety of operating systems or platforms.Additionally, such software may be written using any of a number ofsuitable programming languages and/or conventional programming orscripting tools, and also may be compiled as executable machine languagecode.

In this respect, it should be appreciated that one embodiment of theinvention is directed to a computer readable medium (or multiplecomputer readable media) (e.g., a computer memory, one or more floppydiscs, compact discs, optical discs, magnetic tapes, etc.) encoded withone or more programs that, when executed on one or more computers orother processors, perform methods that implement the various embodimentsof the invention discussed above. The computer readable medium or mediacan be transportable, such that the program or programs stored thereoncan be loaded onto one or more different computers or other processorsto implement various aspects of the present invention as discussedabove.

It should be understood that the term “program” is used herein in ageneric sense to refer to any type of computer code or set ofinstructions that can be employed to program a computer or otherprocessor to implement various aspects of the present invention asdiscussed above. Additionally, it should be appreciated that accordingto one aspect of this embodiment, one or more computer programs thatwhen executed perform methods of the present invention need not resideon a single computer or processor, but may be distributed in a modularfashion amongst a number of different computers or processors toimplement various aspects of the present invention.

Various aspects of the present invention may be used alone, incombination, or in a variety of arrangements not specifically discussedin the embodiments described in the foregoing and is therefore notlimited in its application to the details and arrangement of componentsset forth in the foregoing description or illustrated in the drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. In particular, the variousconcepts related to variable radiation energy and variable radiationintensity may be used in any way, either alone or in any combination, asthe aspects of the invention are not limited to the specificcombinations described herein. Accordingly, the foregoing descriptionand drawings are by way of example only.

Use of ordinal terms such as “first”, “second”, “third”, etc., in theclaims to modify a claim element does not by itself connote anypriority, precedence, or order of one claim element over another or thetemporal order in which acts of a method are performed, but are usedmerely as labels to distinguish one claim element having a certain namefrom another element having a same name (but for use of the ordinalterm) to distinguish the claim elements.

Also, the phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having,” “containing”, “involving”, andvariations thereof herein, is meant to encompass the items listedthereafter and equivalents thereof as well as additional items.

1. A method of displaying image data, the method comprising: obtainingprojection data of an object by exposing an object to radiation at aplurality of view angles and detecting at least some of the radiationexiting the object to form the projection data; operating a computer toreconstruct the projection data at a reconstruction resolution to formimage data comprising a plurality of voxels representing locationswithin the object, each of the plurality of voxels being assigned anassociated intensity indicative of a density of the subject matter atthe respective location; determining a maximum resolution for display,above which variation in intensity between adjacent voxels is notsupported by information in the projection data, the maximum resolutionbeing less than the reconstruction resolution; and displaying the imagedata at or below the maximum resolution.
 2. The method of claim 1,wherein determining the maximum resolution includes determining themaximum resolution based on the number and distribution of the pluralityof view angles from which the image data was obtained.
 3. The method ofclaim 2, operating a computer includes operating a computer toreconstruct the projection data to form a three-dimensional image havingasymmetric resolution, a first resolution along a first dimension of thethree-dimensions being
 4. The method of claim 1, wherein obtainingprojection data includes obtaining projection data by exposing theobject to x-ray radiation, from at least one x-ray source, at aplurality of view angles and detecting at least some of the radiationexiting the object to form the projection data, the projection dataindicative of the amount of x-ray radiation absorbed by the object. 5.The method of claim 4, wherein the object is a human female breast, andwherein obtaining projection data includes obtaining projection data byexposing the female human breast to x-ray radiation from a plurality ofview angles number less than
 30. 6. A method of transforming image datafor display, the image data represented by a plurality of voxels, eachhaving an associated intensity, the method comprising computerimplemented acts of: grouping the plurality of voxels into a pluralityof neighborhoods; and transforming each of the plurality ofneighborhoods into a respective representative voxel having arepresentative intensity computed by: substracting an offset from eachvoxel intensity in the respective neighborhood, averaging the offsetvoxel intensities in the respective neighborhood to obtain an averageintensity, and adding the offset to the average intensity to form therepresentative intensity for the respective neighborhood.
 7. The methodof claim 6, wherein averaging comprises: performing a power function oneach of the offset voxel intensity values to form a plurality of poweroffset voxel intensities; averaging the plurality of power offset voxelintensities to form a power average intensity for the neighborhood; andperforming an inverse power function on the power average intensity toobtain the average intensity.
 8. The method of claim 7, wherein:performing the power function includes squaring each of the offset voxelintensity values to form a plurality of squared offset voxelintensities; and performing an inverse power function includes taking asquare root of the power average intensity.
 9. A method of limiting aviewing angle at which image data may be displayed, the image datareconstructed from projection data obtained by exposing an object toradiation from a plurality of view angles spanning a range having afirst angle boundary and a second angle boundary, the method comprisingcomputer implemented acts of: receiving an indicator from an operator toview the image data from a first angle; displaying the image data on adisplay from the first angle if the first angle is within the rangebounded by the first angle boundary and the second angle boundary; andpreventing the image data from being displayed on the display at thefirst angle if the first angle is outside of the range bounded by thefirst angle boundary and the second angle boundary.
 10. A method ofdisplaying three-dimensional reconstruction image data formed from aplurality of slices, each of the plurality of slices having a voxeldimension N×M×1 in a first direction, a second direction and a thirddirection, respectively, the plurality of slices ordered in the thirddirection, the method comprising computer implemented acts of: defininga window having a voxel width in the third direction equal to apredetermined number of slices of the image data; aligning the windowwith a first plurality of slices to be displayed; transforming thevoxels of the first set of slices within the window into voxels of afirst display slice; providing the display slice to the display forviewing; upon receiving an indication to display image data at asubsequent location in the third direction; moving the window one sliceof the ordered plurality of slices in the third direction to align thewindow with a second set of slices; transforming the voxels of thesecond set of slices within the window into voxels of a second displayslice; and providing the display slice to the display for viewing. 11.The method of claim 10, wherein the window has a voxel width thatincludes a number of slices at least equal to a resolution reductionfactor between a reconstruction resolution of the image data and adisplay resolution of the image data in the third direction.
 12. Themethod of claim 11, wherein the window includes a central portion havinga voxel width equal to the resolution reduction factor and a firstperipheral portion located on a first side of the central portion havinga voxel width of at least one slice, and a second peripheral portionlocated on a second side of the central portion having a voxel width ofat least one slice, and wherein transforming voxels within the windowincludes weighting voxels of the at least one slice within the firstperipheral portion and voxels within the at least one slice within thesecond peripheral portion less than the voxels of slices within thecentral portion.